Many mechanical testing applications require a load distribution to be measured over a physical area. In such applications, it is often necessary to also know the pressure distribution induced by that load over that particular area. The inventors recognized that there is a relationship between pressure distribution and applied strain. Measurements of strain may be used to gain insight into the behavior of mechanical structures under applied loads. One way to do this is to attach a resistive strain gage at a location of interest and interpret the response of the gage. However, this only results in a single point measurement. In order to comprehensively map out the strain response over an area of a structure, it is necessary to install multiple independent resistive strain gages. This effort can be highly time consuming and tedious. Due to the surface area covered by individual resistive strain gages, there is a limited and finite measurement density that can reasonably be achieved. Furthermore, installing a finite number of individual sensors may affect the load distribution and does not provide a homogeneous contact surface. Thus when an application requires a measure of strain over an area of a structure, an array of resistive strain gages is not ideal.
U.S. Pat. No. 5,293,039 describes a pressure sensor in which an optical fiber is sandwiched between several layers to form a sensing pad. In this method, each individual fiber provides only a single pressure measurement. This is an intensity based system and returns a single measurement dependent upon the amount of bend loss that occurs throughout the length of the fiber when a pressure is applied. This method requires that the fiber be woven through a layer of the structure, which is difficult to manufacture. Furthermore, several layers are required, which increases the thickness of the structure and complicates it, further reducing the ability of an external response to couple into the sensing fiber.
The inventors recognized that distributed interferometric fiber optic strain sensing may be utilized to provide a high density strain map of a particular area, and from that strain map, determine a corresponding pressure distribution. Interferometric fiber optic sensing uses an optical fiber as a sensor. A tunable laser source sends light down the fiber and detectors measure the Rayleigh backscatter from the optical fiber. Strain along the fiber axis stretches the spatial frequency of the backscattered light, resulting in a shift of the frequency spectrum reflected from each portion of the fiber. Each scan of the laser interrogates the entire length of sensing fiber, resulting in measurements with micron level spatial resolution. Traditional fiber optic strain sensing provides a one dimensional measurement of strain along the length of the fiber sensor. A map of a two or three dimensional strain field is achieved by a series of fiber passes through the test structure.
Example embodiments include a pressure sensing pad with a flexible planar layer having a two-dimensional pressure sensing area. An optical fiber is attached to and traverses the pressure sensing area of the flexible planar layer. At least one end of the fiber optic strain sensor has a connector configured for connection to an interferometric-based fiber optic interrogation and processing system.
When the connector is connected to the an interferometric-based fiber optic interrogation and processing system and pressure is applied to the pressure sensing pad, a signal from the optical fiber is provided to and processed by the interferometric-based fiber optic interrogation and processing system to determine a two-dimensional pressure map for the two-dimensional sensing area.
Example embodiments also include a system for measuring pressure. A pressure sensing pad includes a layer with a two-dimensional pressure sensing area and an optical fiber attached to and traversing the pressure sensing area of the flexible planar layer the two-dimensional sensing area in a particular configuration. An interferometric-based fiber optic interrogation and processing system is coupled to one end of the optical fiber and is configured, when pressure is applied to the pressure sensing pad, to detect and process reflected light from the fiber optic strain sensor to determine a pressure associated with the two-dimensional sensing area.
In one example implementation, the pressure may be a two-dimensional pressure map. The optical fiber may be configured to be interrogated by the interferometric-based fiber optic interrogation and processing system to provide strain information at multiple different points along the length of the optical fiber which is convertible by the interferometric-based fiber optic interrogation and processing system to the two-dimensional pressure map.
In another example implementation, the optical fiber is a distributed optic strain sensor configured to provide strain information at multiple different points along the length of the optical fiber.
In another example implementation, a particular configuration of the fiber includes at least a serpentine portion.
In another example implementation, a material of the flexible planar layer has a desired elastic modulus range.
In another example implementation, a material of the flexible planar layer reduces friction.
In another example implementation, a surface of the flexible planar layer includes a coating applied thereto that reduces friction.
In another example implementation, a bend sensing pad is formed by including the optical fiber in the pressure sensing pad offset from a central plane of the pressure sensing pad.
In another example implementation, the optical fiber is embedded within a monolithic sheet of epoxy.
Some of the advantages of this technology are that an individual fiber provides a large number of distributed pressure measurements along the fiber that does not depend upon the amount of bend loss that occurs throughout the fiber when a pressure is applied. The fiber does not need to be woven through a layer of the pad. Only a single layer or sheet is needed, thereby providing a thin, flexible, simple, and cost effective structure. The configuration of the single fiber embedded in layer or sheet permits mapping of a two (or three) dimensional strain field.